Abstract: The existence of start-up pressure means that the fluid deviates from the Darey's Law,so the fluid is of the non-linear flow in porous media.Through the measuring of start-up pressure and the study on relation between start-up and permeability as well as the flow experiment of water-flooding,it can be concluded that under the low-velocity flow in porous media,the fluid flow is t non-linear;it ex ists the start-up pressure gradient when the fluid seeps through the low-permeability core.It also shows that the start-up pressure is subject to the permeability of samples.Compared with the permeability under gaseous medium,the lower the permeability of core,the larger falling of permea-bility under water medium and oil medium and the higher the start-up pressure.The displacement velocity has a large effect on the recovery rate of low-permeability reservoir and there exists a optimum displacement velocity.